What are Drum Brakes?
Drum brakes are an older style of braking system that uses a drum design. Decades ago, drum brakes would go on all four wheels, and they were the standard method of slowing down and stopping a vehicle. That is until disc brakes started to be the dominant technology a few decades ago to supplant them mostly.
They earned their name because the brake components are housed in a round drum that rotates along with the vehicle's wheel. Inside this drum is a set of drum brake shoes. When the brake pedal is pressed, it forces the shoes against the drum to slow and then stop the wheel. The shoes are made from a heat-resistant friction material similar to that used on a car's clutch plates.
A drum brake system does not use brake pads as the friction material as brake discs do. Since this braking system does not use brake pads, it does not have calipers that clamp the brake pads against brake rotors. Instead, a drum brake system has a wheel cylinder with pistons that work to push the brake shoes out against the inside of a spinning drum. When contact is made, the friction converts kinetic energy into thermal energy. The drum rotation helps to push the brake shoes and the lining against the drum with increased force. That slows and eventually stops the rotation of the brake drum and the car's wheel.
Today, drum brakes are found mostly on small or low-end cars and only at the rear wheels. Disc brakes go on the front wheels, as they do the majority of the stopping. Many cars, even economy cars, have adopted four-wheel disc brakes.
The Three Types of Drum Brakes
There are three types of drum brakes available. The types vary depending on how the brake shoes are pressed on the drums.
- Leading/ trailing shoe type
- Twin leading shoe type
- Duo-server shoe type
- Leading/Trailing Shoe Type
A leading shoe or primary shoe refers to the brake shoes that move in the same direction as the drum rotation when pressed against it.
The other brake shoe is the trailing shoe or secondary shoe. When the leading shoe is pressed in the same direction as the rotation of the drum, this rotation helps to push the shoes against the brake drum with greater pressure, allowing for more forceful braking power. That is what is known as the servo effect.
The following form part of the main components of leading/trailing shoe type drum brakes:
- Adjuster lever
- Trailing shoe
- Parking brake lever
- Backing plate
- Wheel cylinder
- Return spring
- Leading shoe
- Shoe hold spring
- Twin Leading Shoe Type
This type of drum brake is used mainly for the front brakes of small to mid-sized trucks and has two-wheel cylinders and two leading shoes. Each wheel cylinder presses on one brake shoe. That is so that both of the brake shoes can act as leading brake shoes when the car starts moving forward. And it provides more braking force.
Pistons that displace in both directions form part of the dual twin leading shoe type. This particular arrangement makes it possible for both shoes to act as leading ones, regardless of the car's direction. Dual twin leading shoe type brakes are mainly used for the rear brakes of small to mid-size trucks.
Duo Servo Shoe Type
Duo servo shoe type brakes are mainly used for:
- Parking brakes on passenger cars
- Center brakes on trucks
- Brakes for forklifts
This type features two brake shoes (the primary and the secondary shoes) linked together via an adjuster. Tremendous braking force is generated when strong pressure from the servo effect of the primary shoes is transmitted to the secondary shoe it's linked to.
The Benefits of Drum Brakes
The main reason why drum brakes are still used today is that they are cheaper to manufacture, and they weigh a lot less than disc brakes.
Below are some of the benefits of drum brakes:
They create more heat than disc brakes.
There is a larger surface area on the friction pads. That means the friction pads of a drum brake last longer than the brake pads on calipers of a disc brake system.
Brake shoes wear, and the friction material layer on a worn brake shoe can easily be replaced to allow the part to be re-manufactured.
Unlike disc brakes, drum brakes have a "self-energizing" ability to take advantage of geometry to boost a car's stopping power without increasing the braking force needed.
The Maintenance of Drum Brakes
A lot of heat is created by a braking system, which means the risk of something going wrong is high. Since braking converts kinetic energy into thermal energy, many parts of the braking system are exposed to high temperatures. That can result in a lot of wear and tear, even during normal driving conditions.
On average, the drum housing for drum brakes should last between 150,000 to 200,000 miles under normal driving conditions before the braking friction causes the inside diameter to increase enough to affect the contact with the brake shoes. The brake shoes can typically go for 40,000 miles before they wear out. If your brake shoes are less than two or three millimeters thick, then they need to be replaced.
Some brake components will need to be maintained and replaced over the life of the vehicle; however, there is no set interval because it depends on:
- Driving style.
- Average road conditions.
The best solution for maintaining a drum brake system is to check and replace the friction material, shoes regularly, and other components to avoid vehicle braking system fade or brake fade.
Changing the brake fluid at regular intervals is essential as it is specifically formulated to prevent corrosion of the hydraulic components.
The Difference Between Drum Brakes and Disc Brakes
There are some common principles shared between the two types of braking systems. Firstly friction and heat. When applying friction or resistance to a turning wheel, a vehicle's brakes will cause the car to slow down and come to a complete stop, creating heat as a byproduct.
The rate at which the brakes can slow the car depends on various factors, including:
- Vehicle weight
- Braking force
- Total braking surface area
- How well a braking system converts kinetic energy into thermal energy.
- How quickly the heat is removed from the components.
- Taking the above into consideration, this is where the differences are found between drum brakes and disc brakes.
Another common principle is that both systems operate using the same basic hydraulics; however, the two braking systems perform differently.
- Disc brakes have more stopping power.
- Disc brakes apply quicker for shorter stopping distances and act better as an emergency brake.
- Disc brakes are better at managing heat. Drum brakes retain heat that can reduce a vehicle's braking force.
- Disc brakes perform better on wet roads.
- Disc brakes weigh less.
- Disc brakes have less hardware and are easier to maintain and service.
- Drum brakes are more complex, but they are less expensive to replace.
- Disc brakes are more durable.
- Drum brakes are more prone to grabbing or pulling.
- Disc brakes are self-cleaning.
- There is a larger surface area on the friction pads of drum brakes. That means the friction pads of a drum brake last longer.
- A drum brake system does not use brake pads as the friction material as brake discs do.
The Basics of a Braking System
Here are the basics of a car's braking system in general and how the system works:
Both disc and drum brakes use a hydraulic pressure system. Braking is initiated when your foot presses the brake pedal, known as the mechanical force.
The master cylinder is located beneath the car's hood and close to the engine. Inside the master cylinder, a piston compresses the brake fluid. This compression results in a lot of hydraulic pressure and generates a much larger force than pushing the pedal.
The hydraulic pressure is transferred through the brake fluid in the brake lines to the brake hoses. The brake hoses are flexible tubes that connect the lines with brake assemblies at each of the wheel cylinders.
Lastly, the wheel cylinders work to convert the hydraulic pressure back into mechanical force. The brake friction material is pushed against either the brake drums or brake discs to provide enough stopping power for a car.
Disc and drum brakes are built differently, and both feature their list of advantages and disadvantages. You may find that your vehicle may have front disc and rear drum brakes, or just disc brakes. However, both work as part of a hydraulic braking system. Some base models of vehicles may have brake discs on the front axle and drum brakes on the rear axle. It's due to weight factors and helps keep the costs down to manufacture the vehicle.
Because braking systems are subject to a lot of heat and can be compromised by air, brake dust, moisture, and road grime, it is crucial to do regular brake inspections to avoid brake fade.